1. Field of the Invention
This invention relates to steel cords used as a reinforcement for a rubber article such as pneumatic tire, industrial belt and the like as well as pneumatic radial tires using such steel cords to reinforce a carcass, and more particularly to an improvement of durability in this steel cord.
2. Description of the Related Art
In order to effectively enhance the resistance to corrosion fatigue in steel cords for rubber reinforcement, it is effective to accelerate the penetration of rubber into the inside of the cord, so that there are made various proposals on the twisting structure attaining sufficient rubber penetration or so-called rubber penetration structure.
In tires such as truck and bus tires or light truck tires, however, since the carcass ply is comprised of two- or three-layer twisted strand cords, it is very difficult to completely penetrate rubber into the inner layer of the cord, so that the improvement of the resistance to corrosion fatigue through the rubber penetration structure is slight in such a cord.
On the other hand, as a method of enhancing the resistance to corrosion fatigue of the multi-layer twisted cord, it is known to increase the contact area between filaments constituting the inner layer (core) and filaments constituting the outer layer (sheath) in the cord by forming a compact twisted structure in which the twisting direction and the twisting pitch in each of the layers are the same.
In the steel cord in the running tire, the sectional area of the filament generally decreases due to the fretting wear and the cord strength lowers. If the decrease of sectional area in a part of the filaments inside the cord is violent, such a filament is apt to generate breakage against tensile shock and repetitive bending. When the filament is broken once, the tensile stress of the other filaments increases and hence the fatigue breakage of the cord is promoted. In order to improve the durability of the cord, therefore, it is required to avoid premature breakage of the filament among the filaments constituting the cord and it is desirable that the decrease of strength in the filament becomes uniform over the whole of the cord.